A phytochemical reinvestigation of the aerial parts of Artemisia sieversiana gave a new guaianolide and two known flavones (chrysosplenetin and 5-hydroxy-3',4',6,7-tetramethoxyflavone). Antifungal fractions derived from the chloroform extract of A. annua afforded two cadinane derivatives (arteannuin B and artemisinin), oleanolic acid, beta-sitosterol, stigmasterol, and the four flavones artemetin, bonanzin, eupalitin and chrysosplenetin. Their structures were elucidated by spectral methods. All isolates from the two species were tested in vitro for antifungal activity. Arteannuin B, a main sesquiterpenoid in A. annua, showed antifungal activity against one human (Candida albicans, MIC: 100 micrograms/ml) and four plant pathogenic fungi (Gaeumannomyces graminis var. tritici, Rhizoctonia cerealis, Gerlachia nivalis and Verticillium dahliae, MICs: 150, 100, 150 and 100 micrograms/ml, respectively) whereas others showed no antifungal activity. The MIC value of ketoconazole to C. albicans was 1.0 microgram/ml, and those of triadimefon to G. graminis var. tritici and R. cerealis 150 and 100 micrograms/ml.